The steps of the production of semiconductor silicon integrated circuits must be very clean, because even small amounts of undesirable contaminating impurities can cause complete degradation or malfunction of integrated circuits. Thus, semiconductor silicon wafers must be cleaned between processing steps.
There exist many cleaning processes used in silicon semiconductor production. Wet cleaning processes usually remove contaminants from the silicon wafer surface with special chemical solutions as part of a separate production step. Dry cleaning processes usually remove contaminants by etching the contaminants from the wafer surface within some gas or gas mixture or within a plasma environment. Gettering processes utilize the tendency for contaminants to move towards special traps inside the silicon wafer (areas with a high density of such traps are called "getters") and to stay stable within the traps.
However, wet and dry cleaning processes can only remove contaminants from the silicon wafer surface. Gettering processes can be used only for some types of contaminants (such as iron, copper, nickel etc.).
European Patent Publication EP-A-0749153, assigned to the common owners of the present application, describes a cleaning method which utilizes an electric field in the presence of heat to move positively charged impurity ions from the semiconductor wafer surface and above it towards a negatively charged electrode (called the "collector").
The apparatus of European Patent Publication EP-A-0749153 is schematically illustrated in FIG. 1 to which reference is now briefly made. The electric field is created between the electrodes, one of which, labeled 10, is attached to the wafer 12 to be cleaned. The other electrode (the "collector"), labeled 14, is placed at a distance therefrom. Wafer electrode 10 is connected to a voltage source output Vb and the electrode 14 is connected to another source output Vc, where the potential Vc is more electronegative than the potential Vb. Thus, an electric field is created to move positive ions, labeled 16, from the wafer area toward the collector electrode 14, where positive ions will be absorbed (captured). Typically, the apparatus is placed in a vacuum or in presence of a gas mixture and heated to initiate ion emission from the wafer surface. EP Publication EP-A-0749153, also describes utilizing a plasma between the two electrodes 10 and 14 to strengthen the electric field over the wafer surface.